1. Field of the Invention
This invention relates to a display of a type such as a spectacle type, a goggle type or a helmet type which is mounted on a user.
2. Related Background Art
There are many U.S. patents for head up displays, and many of them relate to helmets for an aircraft. A first type is a primary imaging type as disclosed in U.S. Pat. No. 3,940,204 wherein the image of a CRT or the like is once formed through a lens system. A second type is a virtual image type as disclosed in U.S. Pat. No. 4,026,641 wherein light from an image is not imaged but is directed to an eyeball to thereby form a virtual image. Also, this patent somewhat strengthens the power of the reflecting mirror of a reflecting optical system, eliminates a convex lens usually disposed just in front of an eyeball for enlarging an image, and further sets the reflecting mirror and a CRT so that the screen of the CRT may be inclined toward a man's head, thereby making the apparatus compact. A third type is a prism type as disclosed in U.S. Pat. No. 4,969,724 wherein a prism is utilized to make a reflecting optical system compact. Also, there is U.S. Pat. No. 4,859,030 as a primary imaging type which utilizes a prism, and there is U.S. Pat. No. 4,081,209 as a virtual image type which utilizes a prism.
Now, where a compact and lightweight structure like a goggle type display is sought after, the aforedescribed primary imaging type is good in optical performance, but requires a number of lenses, which leads to bulkiness, and the prism type is compact but becomes heavy. Accordingly, the virtual image type is preferable in respect of compactness and light weight, but is not very good in optical performance. In this point, the aforementioned U.S. Pat. No. 4,026,641 obtains a good optical performance with the reflecting surface of the reflecting optical system as a toric surface and with an original image surface itself as a toric surface. However, to convert the flat image surface of a CRT or the like into a toric surface, it is necessary to use a bundle of glass fibers and the like and this is technically difficult and requires a high cost. Also, in the embodiment shown in this patent, the distance IP from the last surface to the eye point is equal to or greater than 60 mm, and this is a cause which hampers compactness.
In the present invention, as shown in FIG. 2 of the accompanying drawings which will be described later, the point at which the central light beam of the maximum image height light beam in the direction of the longer side of an image intersects the optical axis of an eye is defined as the eye point. Also, the distance from the last effective surface on the optical axis of the eye to the eye point is IP. The last effective surface does not include the glass or the like of a dustproof window.